If you're on a keto diet, then it won't really be possible to look "full." That look is accomplished by already being very lean, and when you have enough glycogen in the muscle to push up against the skin, you look full and tight. No glycogen=lack of fullness, or "flat." But, a physique athlete would typically deplete, and then carb up before a show to get that full and tight look. Having some carbs pre workout would help, and as @Basement_Gainz mentioned, weekly refeeds.